1. Field of the Invention
The present invention relates to a manufacturing method of a disk drive device, a disk drive device and a subassembly of the disk drive device, and particularly to a technique for reducing the amount of unbalance of the disk drive device.
2. Description of the Related Art
Recently, disk drive devices such as HDDs have been dramatically improved in their rotational accuracy by providing fluid dynamic bearings as disclosed in, for example, JP Laid-Open Publication No. 2007-198555. In accordance with this movement, there have been demands for disk drive devices with higher density and larger capacity. For example, in a disk drive device that magnetically records data, a recording disk on which a recoding track is formed is rotated at high speed, and a magnetic head reads or writes the data by tracing the recoding track while keeping a slight gap above the recoding track. In order to provide a disk drive device with high density and large capacity, both the width of recording track and the gap between the magnetic head and the recording disk need to be smaller.
Since a disk drive device rotates a recording disk, the size of the amount of unbalance at a rotating part is critical. It can be assumed that the amount of unbalance is the amount of displacement, from the rotation center, of the center of gravity of an object in a stationary state or in a state of rotation or the amount of unbalance is displacement (axial run-out) from a reference position in a rotation axis direction. A large amount of unbalance of a rotational part of a disk drive device causes great vibration at the time of rotation and is thus likely to cause the displacement (i.e., off-tracking) of a magnetic head from its regular position, for example, from the center of a recording track, when the magnetic head traces over the recording track. As described above, narrowing down the width of a recording track to provide a disk drive device with high density and large capacity results in the effects of off-tracking to be notably shown, which becomes a cause for the increased error rate of reading/writing data. In other words, a large amount of unbalance of a disk drive device hinders a disk drive device from having high density and large capacity.
One of the factors for determining the amount of unbalance of a disk drive device (hereinafter, referred to as DBT) is the amount of unbalance of a hub member on which a recording disk is mounted (hereinafter, referred to as DBH). Another factor is the displacement of the center of gravity from the rotation center, caused by the deviation of a gap formed between the center hole and the outer cylinder part, off to one side when a recording disk is mounted on the outer cylinder part of a hub member through the center hole. In other words, there is the amount of unbalance that is caused by the mounting of a recording disk (hereinafter, referred to as DBD). Therefore, in accordance with an incidental combination of DBH and DBD, a DBT becomes larger when both amounts of unbalance are added up, and the DBT becomes smaller when both amounts of unbalance cancel out each other. As a result, this causes a huge variation in DBT, hindering a disk drive device from having high density and large capacity and the quality stability. Accordingly, efforts have been made toward the reduction of DBH and DBD in order to reduce DBT.
As described above, DBH and DBD need to be reduced in order to reduce DBT. For example, as a means for reducing DBH, one possible option is to improve the part accuracy by adding a process of modifying the amount of unbalance on a hub member. However, the process of modifying the amount of unbalance is not preferable since it requires increased processing time, increased processing cost, and the like. Another possible approach for reducing DBD is to reduce the gap between the center hole of a recording disk and the outer cylinder part of a hub member. However, this reduction in the gap results in a poor workability at the time of mounting a recording disk on a hub member. In other words, the reduction in the gap is not preferable since careful attention is required for assembly tasks so as not to cause the deformation or damage of parts, which results in the reduced efficiency of work. The reduction in the gap between a center hole and an outer cylinder part can be interference in the pursuit of the automatic assembly of a disk drive device.
The inventors and others have reached the conclusion that a reduction in DBT with almost no machining process, such as a modification process on a hub member or a recording disk, allows for the realization of a disk drive device with high density and large capacity without increasing the cost or interfering the pursuit of automatic assembly.